Generally, the spine seems to be the interlocking part of the hinge. I'm thinking therefore that it might be okay to translate spine as "hinge", or 铰炼.

I'm assuming this is a regular mailbox on a roadside or in an apartment building. If that is the case, the spine/hinge is the interlinking part that holds the mailbox door on and enables it to open and close.

HINGES: removing the hinge pin is the first step in taking the door off the hinge. Therefore the spine of the hinge containing the pin should be mounted on the interior side of an exterior door. If the spine is located on the exterior it should be welded into the spine of the hinge or the door should be equipped with a leaf hinge. The leaf hinge has a screw or stud which holds the hinge closed making removal of the door very difficult. (source: http://www.och.umd.edu/resources/safety.htm)